GBA Development Kits For Sale
I am now selling them as a bundle with 1 linker and 2 flash carts. You will receive 2 128m carts, a USB (and parallel port) capable linker, and a USB cable.
You can read more about these here.
Note: These are 128m (128 megabits = 16 megabytes) cartridges, so please check your application to be sure these will work.
The official software is supported under Windows XP only. If you want to see how to get this running under the free VMWare player, please see this post. I am also working on some software to use this under OSX and Windows 7/8. It works, but it has not been released publically, yet.